Sympathetic eruptions on the Sun have been observed for several decades, but the mechanisms by which one eruption can trigger another remain poorly understood. We present a three-dimensional MHD ...simulation that suggests two possible magnetic trigger mechanisms for sympathetic eruptions. We consider a configuration that contains two coronal flux ropes located within a pseudo-streamer and one rope located next to it. A sequence of eruptions is initiated by triggering the eruption of the flux rope next to the streamer. The expansion of the rope leads to two consecutive reconnection events, each of which triggers the eruption of a flux rope by removing a sufficient amount of overlying flux. The simulation qualitatively reproduces important aspects of the global sympathetic event on 2010 August 1 and provides a scenario for the so-called twin filament eruptions. The suggested mechanisms are also applicable for sympathetic eruptions occurring in other magnetic configurations.
Abstract
One of the striking observations from the Parker Solar Probe (PSP) spacecraft is the prevalence in the inner heliosphere of large amplitude, Alfvénic magnetic field reversals termed
...switchbacks
. These
δ
B
R
/
B
∼
(
1
) fluctuations occur over a range of timescales and in
patches
separated by intervals of quiet, radial magnetic field. We use measurements from PSP to demonstrate that patches of switchbacks are localized within the extensions of plasma structures originating at the base of the corona. These structures are characterized by an increase in alpha particle abundance, Mach number, plasma
β
and pressure, and by depletions in the magnetic field magnitude and electron temperature. These intervals are in pressure balance, implying stationary spatial structure, and the field depressions are consistent with overexpanded flux tubes. The structures are asymmetric in Carrington longitude with a steeper leading edge and a small (∼1°) edge of hotter plasma and enhanced magnetic field fluctuations. Some structures contain suprathermal ions to ∼85 keV that we argue are the energetic tail of the solar wind alpha population. The structures are separated in longitude by angular scales associated with supergranulation. This suggests that these switchbacks originate near the leading edge of the diverging magnetic field funnels associated with the network magnetic field—the primary wind sources. We propose an origin of the magnetic field switchbacks, hot plasma and suprathermals, alpha particles in interchange reconnection events just above the solar transition region and our measurements represent the extended regions of a turbulent outflow exhaust.
Low-speed ( 450 km s−1) solar wind is widely considered to originate from streamer loops that intermittently release their contents into the heliosphere, in contrast to high-speed wind, which has its ...source in large coronal holes. To account for the presence of slow wind far from the heliospheric current sheet (HCS), it has been suggested that "pseudostreamers" rooted between coronal holes of the same polarity continually undergo interchange reconnection with the adjacent open flux, producing a wide band of slow wind centered on the separatrix/plasma sheet that extends outward from the pseudostreamer cusp. Employing extreme-ultraviolet images and potential-field source-surface extrapolations, we have identified 10 Earth-directed pseudostreamers during 2013-2016. In situ measurements show wind speeds ranging from ∼320 to ∼600 km s−1 in the days immediately preceding and following the predicted pseudostreamer crossings, with the proton densities and O7+/O6+ ratios tending to be inversely correlated with the bulk speed. We also identify examples of coronal holes that straddle the solar equator and give rise to wind speeds of order 400 km s−1. Our results support the idea that the bulk of the slow wind observed more than a few degrees from the HCS originates from just inside coronal holes.
A sequence of apparently coupled eruptions was observed on 2010 August 1-2 by Solar Dynamics Observatory and STEREO. The eruptions were closely synchronized with one another, even though some of them ...occurred at widely separated locations. In an attempt to identify a plausible reason for such synchronization, we study the large-scale structure of the background magnetic configuration. The coronal field was computed from the photospheric magnetic field observed at the appropriate time period by using the potential field source-surface model. We investigate the resulting field structure by analyzing the so-called squashing factor calculated at the photospheric and source-surface boundaries, as well as at different coronal cross-sections. Using this information as a guide, we determine the underlying structural skeleton of the configuration, including separatrix and quasi-separatrix surfaces. Our analysis reveals, in particular, several pseudo-streamers in the regions where the eruptions occurred. Of special interest to us are the magnetic null points and separators associated with the pseudo-streamers. We propose that magnetic reconnection triggered along these separators by the first eruption likely played a key role in establishing the assumed link between the sequential eruptions. The present work substantiates our recent simplified magnetohydrodynamic model of sympathetic eruptions and provides a guide for further deeper study of these phenomena. Several important implications of our results for the S-web model of the slow solar wind are also addressed.
Context.
Parker Solar Probe (PSP) measures the magnetic field and plasma parameters of the solar wind at unprecedentedly close distances to the Sun. These data provide great opportunities to study ...the early-stage evolution of magnetohydrodynamic (MHD) turbulence in the solar wind.
Aims.
In this study, we make use of the PSP data to explore the nature of solar wind turbulence focusing on the Alfvénic character and power spectra of the fluctuations and their dependence on the distance and context (i.e., large-scale solar wind properties), aiming to understand the role that different effects such as source properties, solar wind expansion, and stream interaction might play in determining the turbulent state.
Methods.
We carried out a statistical survey of the data from the first five orbits of PSP with a focus on how the fluctuation properties at the large MHD scales vary with different solar wind streams and the distance from the Sun. A more in-depth analysis from several selected periods is also presented.
Results.
Our results show that as fluctuations are transported outward by the solar wind, the magnetic field spectrum steepens while the shape of the velocity spectrum remains unchanged. The steepening process is controlled by the “age” of the turbulence, which is determined by the wind speed together with the radial distance. Statistically, faster solar wind has higher “Alfvénicity,” with a more dominant outward propagating wave component and more balanced magnetic and kinetic energies. The outward wave dominance gradually weakens with radial distance, while the excess of magnetic energy is found to be stronger as we move closer toward the Sun. We show that the turbulence properties can significantly vary from stream to stream even if these streams are of a similar speed, indicating very different origins of these streams. Especially, the slow wind that originates near the polar coronal holes has much lower Alfvénicity compared with the slow wind that originates from the active regions and pseudostreamers. We show that structures such as heliospheric current sheets and velocity shears can play an important role in modifying the properties of the turbulence.
Context.
The launch of Parker Solar Probe (PSP) in 2018, followed by Solar Orbiter (SO) in February 2020, has opened a new window in the exploration of solar magnetic activity and the origin of the ...heliosphere. These missions, together with other space observatories dedicated to solar observations, such as the Solar Dynamics Observatory, Hinode, IRIS, STEREO, and SOHO, with complementary in situ observations from WIND and ACE, and ground based multi-wavelength observations including the DKIST observatory that has just seen first light, promise to revolutionize our understanding of the solar atmosphere and of solar activity, from the generation and emergence of the Sun’s magnetic field to the creation of the solar wind and the acceleration of solar energetic particles.
Aims.
Here we describe the scientific objectives of the PSP and SO missions, and highlight the potential for discovery arising from synergistic observations. Here we put particular emphasis on how the combined remote sensing and in situ observations of SO, that bracket the outer coronal and inner heliospheric observations by PSP, may provide a reconstruction of the solar wind and magnetic field expansion from the Sun out to beyond the orbit of Mercury in the first phases of the mission. In the later, out-of-ecliptic portions of the SO mission, the solar surface magnetic field measurements from SO and the multi-point white-light observations from both PSP and SO will shed light on the dynamic, intermittent solar wind escaping from helmet streamers, pseudo-streamers, and the confined coronal plasma, and on solar energetic particle transport.
Methods.
Joint measurements during PSP–SO alignments, and magnetic connections along the same flux tube complemented by alignments with Earth, dual PSP–Earth, and SO-Earth, as well as with STEREO-A, SOHO, and BepiColumbo will allow a better understanding of the in situ evolution of solar-wind plasma flows and the full three-dimensional distribution of the solar wind from a purely observational point of view. Spectroscopic observations of the corona, and optical and radio observations, combined with direct in situ observations of the accelerating solar wind will provide a new foundation for understanding the fundamental physical processes leading to the energy transformations from solar photospheric flows and magnetic fields into the hot coronal plasma and magnetic fields and finally into the bulk kinetic energy of the solar wind and solar energetic particles.
Results.
We discuss the initial PSP observations, which already provide a compelling rationale for new measurement campaigns by SO, along with ground- and space-based assets within the synergistic context described above.
Coronal cells are visible at temperatures of ~1.2 MK in Fe XII coronal images obtained from the Solar Dynamics Observatory and Solar Terrestrial Relations Observatory spacecraft. We show that near a ...filament channel, the plumelike tails of these cells bend horizontally in opposite directions on the two sides of the channel like fibrils in the chromosphere. Because the cells are rooted in magnetic flux concentrations of majority polarity, these observations can be used with photospheric magnetograms to infer the direction of the horizontal field in filament channels and the chirality of the associated magnetic field. This method is similar to the procedure for inferring the direction of the magnetic field and the chirality of the fibril pattern in filament channels from H alpha observations. However, the coronal cell observations are easier to use and provide clear inferences of the horizontal field direction for heights up to ~50 Mm into the corona.
Context.
Turbulence dominated by large-amplitude, nonlinear Alfvén-like fluctuations mainly propagating away from the Sun is ubiquitous in high-speed solar wind streams. Recent studies have ...demontrated that slow wind streams may also show strong Alfvénic signatures, especially in the inner heliosphere.
Aims.
The present study focuses on the characterisation of an Alfvénic slow solar wind interval observed by Solar Orbiter between 14 and 18 July 2020 at a heliocentric distance of 0.64 AU.
Methods.
Our analysis is based on plasma moments and magnetic field measurements from the Solar Wind Analyser (SWA) and Magnetometer (MAG) instruments, respectively. We compared the behaviour of different parameters to characterise the stream in terms of the Alfvénic content and magnetic properties. We also performed a spectral analysis to highlight spectral features and waves signature using power spectral density and magnetic helicity spectrograms, respectively. Moreover, we reconstruct the Solar Orbiter magnetic connectivity to the solar sources both via a ballistic and a potential field source surface (PFSS) model.
Results.
The Alfvénic slow wind stream described in this paper resembles, in many respects, a fast wind stream. Indeed, at large scales, the time series of the speed profile shows a compression region, a main portion of the stream, and a rarefaction region, characterised by different features. Moreover, before the rarefaction region, we pinpoint several structures at different scales recalling the spaghetti-like flux-tube texture of the interplanetary magnetic field. Finally, we identify the connections between Solar Orbiter in situ measurements, tracing them down to coronal streamer and pseudostreamer configurations.
Conclusions.
The characterisation of the Alfvénic slow wind stream observed by Solar Orbiter and the identification of its solar source are extremely important aspects for improving the understanding of future observations of the same solar wind regime, especially as solar activity is increasing toward a maximum, where a higher incidence of this solar wind regime is expected.
The spectacular prominence eruption and CME of 31 August 2007 are analyzed stereoscopically using data from NASA’s twin
Solar Terrestrial Relations Observatory
(STEREO) spacecraft. The technique of ...tie pointing and triangulation (T&T) is used to reconstruct the prominence (or filament when seen on the disk) before and during the eruption. For the first time, a filament barb is reconstructed in three-dimensions, confirming that the barb connects the filament spine to the solar surface. The chirality of the filament system is determined from the barb and magnetogram and confirmed by the skew of the loops of the post-eruptive arcade relative to the polarity reversal boundary below. The T&T analysis shows that the filament rotates as it erupts in the direction expected for a filament system of the given chirality. While the prominence begins to rotate in the slow-rise phase, most of the rotation occurs during the fast-rise phase, after formation of the CME begins. The stereoscopic analysis also allows us to analyze the spatial relationships among various features of the eruption including the pre-eruptive filament, the flare ribbons, the erupting prominence, and the cavity of the coronal mass ejection (CME). We find that erupting prominence strands and the CME have different (non-radial) trajectories; we relate the trajectories to the structure of the coronal magnetic fields. The possible cause of the eruption is also discussed.
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